Synthesis and crystal structures of cerium(IV) complexes with 8-quinolinolate and amine bis(phenolate) ligands

Article abstract of DOI:10.1016/j.ica.2012.11.020

Two cerium(IV) complexes of Ce(OQ)₄ (1) (HOQ = 8-hydroxyquinoline) and Ce(L)₂·DME (2·DME) (LH ₂ = Me₂NCH₂CH₂CH ₂N{CH₂-(3,5-^tBu₂-C₆H ₂-2-OH)}₂) have been synthesized and characterized by elemental analysis, infrared spectra and X-ray crystallography. Complex 1 is a new type of homoleptic 8-quinolinolate lanthanide complex and its overall geometry is like a four-wing-waterwheel. Complex 2 has two tridentate (ONO) amine bis(phenolate) ligands with their terminal N atoms in side-arms without participating in coordination. Both of 1 and 2 can catalyze the oxidation of benzyl alcohol. The conversion of benzyl alcohol by 2 is higher than that by 1 under comparable conditions.

Full text of DOI:10.1016/j.ica.2012.11.020

Inorganica Chimica Acta 397 (2013) 69–74
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Inorganica Chimica Acta
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Synthesis and crystal structures of cerium(IV) complexes with 8-quinolinolate
and amine bis(phenolate) ligands
⇑
Li Li, Fugen Yuan , Tingting Li, Yuan Zhou, Manman Zhang
School of Chemistry and Biochemistry, University of Science and Technology of Suzhou, Suzhou 215009, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two cerium(IV) complexes of Ce(OQ)4 (1) (HOQ = 8-hydroxyquinoline) and Ce(L)2ꢀDME (2ꢀDME) (LH2
=
Me2NCH2CH2CH2N{CH2-(3,5-tBu2-C6H2-2-OH)}2) have been synthesized and characterized by elemental
analysis, infrared spectra and X-ray crystallography. Complex 1 is a new type of homoleptic 8-quinolin-
olate lanthanide complex and its overall geometry is like a four-wing-waterwheel. Complex 2 has two
tridentate (ONO) amine bis(phenolate) ligands with their terminal N atoms in side-arms without partic-
ipating in coordination. Both of 1 and 2 can catalyze the oxidation of benzyl alcohol. The conversion of
benzyl alcohol by 2 is higher than that by 1 under comparable conditions.
Received 8 September 2012
Received in revised form 21 November 2012
Accepted 23 November 2012
Available online 3 December 2012
Keywords:
Cerium(IV)
8-Quinolinolate
Amine bis(phenolate)
Oxidation
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
and solubility of this complex were determined. Its structure has
not been established up to date. Herein we report the synthesis
Cerium ion is special for its readily accessible tetravalent oxida-
tion state as well as facile +4/+3 redox couple. Due to its unique re-
dox chemistry, cerium(IV) complexes have been applied as
versatile oxidative reagents in organic synthesis [1], photocatalytic
oxidation [2], electrochemistry [3] and biochemistry [4]. There is
an increasing demand for new and well-defined cerium(IV) spe-
cies. Many types of Ce(IV) complexes have been reported such as
[{(Ph2SiO)2O}{K(THF)2}]2Ce(OtBu)2 [5], [{(Ph2SiO)2O}2{(DME)KOt
Bu}{(Ph2SiO2)K}Ce]2 [5], p-tBu-calix[4](OMe)2(O)2Ce(acac)2 [6],
and crystal structures of Ce(OQ)4 (1) (HOQ = 8-hydroxyquinoline)
and Ce(L)2ꢀDME (2ꢀDME) (LH2 = Me2NCH2CH2CH2N{CH2-(3,5-tBu2-
C6H2-2-OH)}2). Complex
1 is a new type of homoleptic 8-
quinolinolate lanthanide complex, and the cerium(IV) ion in
complex 2 has low coordination number, because the terminal N
atoms in side-arms do not participate in coordination. Finally,
the catalytic activity of 1 and 2 for oxidation of benzyl alcohol
was tested.
Ce3(l l
3-OtBu)2( -OtBu)3(OtBu)6 [7], Ce(L1)4 (L1 = 2,2,7-trimethyl-
2. Experimental
t
3ꢁ
3,5-octanedione) [8], Ce(NN03)I [NN033ꢁ = N(CH2CH2NSiMe2 Bu)3
]
[9], CeCl[N(SiMe3)2]3 [10], Ce(salfen)(OtBu)2 (Salfen = ferrocene
Schiff base) [11], Ce(L2)X (X = Cl [12], NO3 [12], BPh4 [13], N3
[13]; L2H3 = N{CH2CH2N = CH(C6H2-tBu2-3,5-OH-2)}3), Ce(L3)2 (L3
H2 = N,N0-bis(3,5-di-tert-butylsalicylidene)ethylenediamine) [14],
2.1. General
Tetrahydrofuran (THF) and 1,2-dimethoxyethane (DME) were
distilled from sodium/benzophenone. Ceric ammonium nitrate
(CAN) was dried at 110 °C. Sodium tert-butoxide (tBuONa) was pre-
pared by reaction of tert-butanol with sodium hydride (NaH) in
THF. Ce(SO4)2ꢀ4H2O and 8-hydroxyquinoline were purchased from
commercial sources and used as received without further purifica-
tion. Amine bis(phenol) LH2 was synthesized according to the liter-
ature procedure [22]. Carbon, hydrogen and nitrogen analyses
were carried out by direct combustion on an EA1110-CHNSO ele-
mental analyzer. Cerium(IV) elemental analysis was carried out
by complexometric titration as normal except that an excess of
ascorbic acid was added to the solution before the titration to re-
duce all Ce4+ to Ce3+ [23]. The IR spectra were recorded on a Perkin
Elmer Spectrum BXII spectrometer. Melting point was taken in a
sealed argon-filled capillary and uncorrected.
Ce(L4)2ꢀCHCl3
(L4H2 = N,N0-bis(2-hydroxybenzyl)ethane-1,2-
diamine) [15], Ce(L5)4 (L5H = C{(NiPr)CHCHN}CH2CMe2OH) [16],
Ce{C8H4(SiiPr3-1,4)2}2 [17], (C5H5)3CeCl [18], [p-MeOC6H4C(NSiM-
e3)2]3CeCl [18], etc. It is noteworthy that the ligands are mainly
O/N species, and Schiff bases are usually used during the synthesis
of Ce(IV) aryloxide complexes.
Actually, 8-hydroxyquinoline and amine bis(phenol) are chem-
ically more stable than Schiff bases. They have been widely used in
coordination with lanthanide metals [19,20]. The formation of
Ce(IV) 8-quinolinolate complex has been known to analytical
chemists for a long time [21]. However, only the composition
⇑
Corresponding author. Tel.: +86 512 68418483; fax: +86 512 68418935.
E-mail address: yuanfugensuzhou@163.com (F. Yuan).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.11.020

70
L. Li et al. / Inorganica Chimica Acta 397 (2013) 69–74
Table 1
2.2. Synthesis and characterization of 1
Summary of the crystallographic data for 1 and 2.
To an aqueous solution (20 mL) of Ce(SO4)2ꢀ4H2O (0.392 g,
0.970 mmol) was added an alcohol solution (20 mL) of 8-hydroxy-
quinoline (0.564 g, 3.884 mmol). Brown precipitate produced
immediately. The mixture was stirred, and a NaOH solution
(1.0 mol/L) was slowly dropped into it until pH value was about
7. During the process, more precipitate was produced. Black brown
powders of 1 (0.516 g, 74.2%) could be obtained by filtration. Crys-
tals suitable for X-ray diffraction analysis were obtained by recrys-
tallization of the powder in a 2-methoxyethanol solution at 25 °C.
Anal. Calc. for C36H24CeN4O4: C, 60.32; H, 3.35; N, 7.82; Ce, 19.56.
Found: C, 59.60; H, 3.31; N, 7.78; Ce, 19.41%. M.p. >250 °C. IR
Complex
1
2ꢀDME
C74H122CeN4O6
1303.88
296
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
Crystal size (mm)
a (Å)
C
36H24CeN4O4
716.71
296
monoclinic
P2(1)/n
0.18 ꢂ 0.14 ꢂ 0.12
9.986(4)
9.778(4)
16.200(6)
90
106.783(4)
90
1514.4(10)
2
triclinic
ꢀ
P1
0.35 ꢂ 0.23 ꢂ 0.18
18.4201(14)
21.3300(16)
21.4870(17)
100.4890(10)
96.2760(10)
113.2860(10)
7469.8(10)
4
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
(KBr, cmꢁ1
) 3434(s), 2364(m), 2346(m), 1599(m), 1493(m),
1461(m), 1377(m), 1315(s), 1270(m), 1234(s), 1105(s), 823(w),
786(w), 733(m), 608(w), 508(w), 484(w).
V (Å3)
Z
Dcalc (g cmꢁ3
F(000)
)
1.572
716
1.550
1.159
2800
0.659
l
(mmꢁ1
)
h range for data collection 2.13–27.55
(°)
1.23–27.45
2.3. Synthesis and characterization of 2
Total No. of reflections
No. of unique reflections
Data/restraints/
parameters
Index range
12761
3468
3468/290/204
65702
33385
33385/4060/1516
The synthesis was carried out under an atmosphere of argon
using Schlenk techniques because the precursor Ce(OtBu)4 is very
sensitive toward moisture and air. A THF solution of Ce(OtBu)4
(2.16 mmol) was prepared in situ from ceric ammonium nitrate
(CAN) (1.184 g, 2.16 mmol) and tBuONa (12.96 mmol) in THF
according to the literature method [23]. It was added into a THF
solution of LH2 (2.328 g, 4.32 mmol), the mixture immediately
turned from orange to brown black. It was stirred for a day, and
then evaporated to dryness. DME solvent was condensed in to dis-
solve the residue. After centrifugation, a clear black solution was
got. It was frozen in the refrigerator for 3 d. Black crystals of 2ꢀDME
(1.168 g, 41.7%) were produced. M.p. 226.8 °C. Anal. Calc. for C74
H122CeN4O6: C, 68.20; H, 9.37; N, 4.30; Ce, 10.76. Found: C,
68.00; H, 9.32; N, 4.22; Ce, 10.80%. IR (KBr, cmꢁ1) 3435(m),
2915(s), 2865(m), 2813(w), 2777(w), 1624(w), 1466(s), 1439(m),
1413(m), 1390(w), 1360(w), 1307(w), 1258(s), 1239(s), 1204(w),
1168(w), 1129(w), 876(w), 834(m), 807(w), 745(m), 535(m),
455(w).
ꢁ12 6 h 6 12
–11 6 k 6 12
–20 6 l 6 20
R1 = 0.0564,
wR2 = 0.1344
R1 = 0.0811,
wR2 = 0.1470
1.126
ꢁ23 6 h 6 23
–27 6 k 6 27
–27 6 l 6 27
R1 = 0.0537,
wR2 = 0.1230
R1 = 0.1008,
wR2 = 0.1494
1.053
Final R indices [I > 2
r(I)]
R indices (all data)
Goodness-of-fit (GOF)
Residual peaks (e/Å3)
0.816, ꢁ0.881
1.320, ꢁ0.863
2.4. Determination of crystal structures
Data collection for complexes 1 and 2 were performed on a
APEX II 4 K CCD area detector equipped with a graphite monochro-
mated Mo Ka radiation (k = 0.71073 Å) by using the x-scan mode
at room temperature. All absorption corrections were applied
using the CRYSTALCLEAR program [24]. The structures were solved by
direct methods for 1 and patterson methods for 2, respectively.
The cerium metal atoms were located from the e-maps, and other
non-hydrogen atoms were derived from the successive difference
Fourier peaks. The hydrogen atoms of the ligands were positioned
geometrically, and allowed to ride on their parent C atoms. The
hydrogen atoms of complex 1 were not located due to the orienta-
tional disorder of 8-quinolinolates [25]. Two structures were re-
fined on F2 by full-matrix least-squares using the SHELXTL-97
program package [26]. Summary of the crystallographic data for
1 and 2 are given in Table 1.
Fig. 1. Molecular structure of complex 1 showing 30% probability ellipsoids with H
atoms omitted for clarity.
2.5. Catalytic oxidation of benzyl alcohol
Complex, PhCH2OH and NaBrO3 were charged to a round bot-
tomed flask. The mixture was stirred in a thermostat bath for a cer-
tain time. It was cooled down and extracted for three times with
ethylacetate. The extracted sample was analyzed by a gas chro-
matograph (GC) to determine the contents of benzaldehyde (with
nitrobenzene as internal standard) and benzoic acid (with di-
methyl phthalate as internal standard).
3. Results and discussion
3.1. Synthesis and crystal structure of 1
8-Hydroxyquinoline is a classical chelating reagent for lantha-
nides. Although complexes of Ln(OQ)3 (Ln = Sc, La, Pr, Eu, Yb) were
synthesized early [27], they are unfortunately insoluble in any

L. Li et al. / Inorganica Chimica Acta 397 (2013) 69–74
71
Table 2
tunately, complex 1 is soluble in 2-methoxyethanol and can be
crystallized in the solvent. The molecular structure of 1, deter-
mined by X-ray crystallography, is shown in Fig. 1. The selected
bond lengths and angles are listed in Table 2. The reaction process
is expressed in Scheme 1.
Selected bond lengths (Å) and angles (°) for complex 1.
Ce(1)–O(1)
Ce(1)–O(2)
Ce(1)–N(1)
Ce(1)–N(2)
O(10)–Ce(1)–O(20)
O(10)–Ce(1)–O(2)
O(20)–Ce(1)–O(2)
O(10)–Ce(1)–C(10)
2.263(10)
2.263(10)
2.612(12)
2.644(10)
57.1(4)
55.0(4)
84.6(4)
154.3(4)
Ce(1)–O(10)
2.193(9)
2.222(9)
2.612(12)
2.644(10)
122.9(4)
125.0(4)
95.4(4)
Ce(1)–O(20)
Ce(1)–N(10)
Ce(1)–N(20)
O(10) –Ce(1)–O(20)
O(10)–Ce(1)–O(2)
O(20)–Ce(1)–O(2)
O(2)–Ce(1)–C(10)
As shown in Fig. 1, a cerium ion is surrounded by four 8-
quinolinolate ligands. Four central axes of quinolinolate ligands
are located in a plane with the cerium ion. The four ligands approx-
imately divide a circle into four equipartitions, as the dihedral
angles between the neighbor 8-quinolinolate planes are 89.5°.
The overall geometry is like a four-wing-waterwheel. Each 8-
quinolinolate group is coordinated bisdentately to the cerium ion
via nitrogen and oxygen atoms. The coordination number of
cerium is eight. The 8-quinolinolate ligands in complex 1 display
orientational disorder with 50% occupancy of both disordered
forms (Fig. 2). This phenomenon is also observed in other 8-
quinolinolate metal complexes of Ho3(OQ)9ꢀHOQ [25], Hf(OQ)4ꢀ
C3H7NOꢀH2O [30] and LaNi2(OQ)7 [19f].
106.4(4)
Symmetry code: 2 ꢁ x, 2 ꢁ y, ꢁz.
N
O
N
N
NaOH
O
O
Ce(SO4)2 + 4
N
Ce
The selected bond lengths and angles for complex 1 are pre-
sented in Table 2. The oxidation state of cerium was calculated
to be 4.1 according to the bond valence sum [31]. The Ce–O bond
lengths (2.193(9)–2.263(10) Å) are consistent with those in
Ce(L3)2 (2.237(3)–2.258(4) Å) [14] and Ce(L4)2ꢀCHCl3 (2.188(4)–
2.239(4) Å) [15]. The Ce–N bond lengths (2.612(12)–2.644(10) Å)
are comparable with those in Ce(L2)NO3 (2.569(2)–2.808(2) Å)
[12] and Ce(L4)2ꢀCHCl3 (2.603(4)–2.628(4) Å) [15].
O
OH
N
Scheme 1.
solvents. In 2005, Deacon’s group obtained accidentally the first
homoleptic Ho3(OQ)9ꢀHOQ [25], which is the only structure of
homoleptic 8-quinolinolate lanthanide complex to the best of our
knowledge. The coordination of 8-quinolinolate with Ce(IV) has
not been reported yet.
3.2. Synthesis and crystal structure of 2
Complex 2 was synthesized by protolysis of Ce(OtBu)4 with
amine bis(phenol) (LH2). Crystals suitable for X-ray diffraction
analysis were obtained from the DME solution. The molecular
structure of 2 is shown in Fig. 3, and the selected bond lengths
and angles are listed in Table 3. The reaction process is expressed
in Scheme 2.
According to X-ray results (Fig. 3), a cerium ion is coordinated
by two tridentate (ONO) amine bis(phenolate) ligands. Two termi-
nal tertiary N atoms in the side arms do not participate in coordi-
nation with the cerium. This may account for the steric hindrance
of bulky tert-butyl substituents. The coordination number of cer-
ium ion is six: four oxygen atoms and two nitrogen atoms. The
When Ce(SO4)2 aqueous solution was mixed with 8-hydroxy-
quinoline, the color of the mixture changed from yellow to black
immediately, and black powders precipitated at the same time,
which indicated that the reaction occurred. When the pH value
of the mixture was adjusted with a NaOH solution from acidity
to neuter, more black precipitates could be obtained. When pH ar-
rived 7, the yield reached maximum of 74.2%. The elemental anal-
yses and the IR spectra were in agreement with the formation of
complex 1. The IR absorption bands at 1376, 1460, 1493, and
1599 cmꢁ1 are assigned to C@N and C@C stretching vibration fre-
quencies in 8-quinolinolate ligand [28,19c], and the absorptions
at 484 and 733 cmꢁ1 are attributed to the Ce–O vibration [29]. For-
Fig. 2. The two disordered forms (equal occupancy) of ligand in complex 1.

72
L. Li et al. / Inorganica Chimica Acta 397 (2013) 69–74
Fig. 3. Molecular structure of complex 2 showing 30% probability ellipsoids with H atoms omitted for clarity.
ionic radius difference between six-coordinate Ce4+ (0.87 Å) and
Table 3
eight-coordinate Ce4+ (0.97 Å) was taken into account [32]. The
Ce–N bond lengths (average 2.597 Å) are a little shorter than that
in eight-coordinate Ce(L2)Cl (2.774(2) Å) [12] when the ionic ra-
dius difference was taken into account, but comparable with that
in seven-coordinate Ce(L2)BPh4 (2.6548(19) Å) [13]. Three atoms
of N(1), Ce(1) and N(2) are nearly in a line (178.29(10)°). The bite
angles of O(1)–Ce(1)–O(2) and O(3)–Ce(1)–O(4) are typical for
Ce(IV) complexes [12–14].
Selected bond lengths (Å) and angles (°) for complex 2.
Ce(1)–O(1)
2.140(3)
2.139(3)
2.598(3)
148.6(10)
73.8(10)
75.1(11)
95.0(11)
149.7(11)
75.0(10)
103.6(10)
178.3(10)
Ce(1)–O(2)
2.160(3)
2.142(3)
2.596(3)
92.1(11)
106.3(11)
104.9(11)
90.1(11)
106.7(10)
99.0(11)
74.7(10)
Ce(1)–O(3)
Ce(1)–O(4)
Ce(1)–N(1)
Ce(1)–N(2)
O(1)–Ce(1)–O(2)
O(1)–Ce(1)–N(1)
O(2)–Ce(1)–N(1)
O(3)–Ce(1)–O(1)
O(3)–Ce(1)–O(4)
O(3)–Ce(1)–N(2)
O(4)–Ce(1)–N(1)
N(1)–Ce(1)–N(2)
O(1)–Ce(1)–O(4)
O(1)–Ce(1)–N(2)
O(2)–Ce(1)–N(2)
O(3)–Ce(1)–O(2)
O(3)–Ce(1)–N(1)
O(4)–Ce(1)–O(2)
O(4)–Ce(1)–N(2)
3.3. Catalytic activity of 1 and 2
The catalytic oxidation of benzyl alcohol in the presence of 1
and 2 was tested. In the reaction complexes of 1 and 2 acted as
the catalyst, and sodium bromate (NaBrO3) did as the oxidant.
Although complexes of 1 and 2 are reactive toward benzyl alcohol,
the reactions are stoichiometric. The combination of Ce(IV) com-
plex with sodium bromate leads to the recycle of Ce(IV) species
from the Ce(III) compound (Eq. (1)) [1a]. It is noteworthy that so-
dium bromate itself, without Ce(IV) catalyst, is unable to oxidize
benzyl alcohol. Both of 1 and 2 in the presence of 1 equivalent of
coordination polyhedron around cerium atom can be described as
a distorted octahedron (Fig. 4).
The oxidation state of cerium in complex 2 was calculated to be
4.4 according to the bond valence sum [31]. The Ce–O bond lengths
(2.139(3)–2.160(3) Å) are consistent with those in Ce(L5)4
(2.130(4)–2.140(4) Å) [16]. They are also comparable with those
in eight-coordinate Ce(L3)2 (2.237(3)–2.258(4) Å) [14] when the
N
N
N
OH
OH
O
O
O
O
N
Ce(OtBu)4 + 2
Ce
N
N
Scheme 2.

L. Li et al. / Inorganica Chimica Acta 397 (2013) 69–74
73
Funded by the Priority Academic Program Development of Jiangsu
Higher Education Institutions (PAPD).
Appendix A. Supplementary material
CCDC 872689 and CCDC 873855 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.ica.2012.11.020.
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Table 4
Oxidation of benzyl alcohol catalyzed by 1 and 2.
cat. (4-10 mol%)
NaBrO3 (1 eqiv)
CH2OH
+
CHO
COOH
no solvent
Entry Cat. n(cat./PhCH2OH)
Temp.
(°C)
Yield of
Yield of
PhCHO (%)
(molar ratio)
PhCOOH (%)
(b) R.T. Kovacic, J.T. Welch, S.J. Franklin, J. Am. Chem. Soc. 125 (2003) 6656;
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2291.
1
2
3
4
5
6
1
1
1
1
2
2
0.04
0.10
0.10
0.15
0.10
0.10
80
80
100
100
60
0.0
0.6
4.4
10.5
27.6
61.5
0.0
4.3
6.4
7.6
5.2
7.3
80
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Acknowledgments
Financial support for this study was provided by Jiangsu Key
Laboratory for the Environment Functional Materials and A Project

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